The invention relates to a method of reducing the reflectance of a transparent viewing screen.
The invention also relates to a viewing screen with reduced reflectance, the outer surface of the screen being provided with an anti-reflective coating.
The viewing screen may be the screen of a cathode-ray tube, for example a television picture tube, but alternatively the screen of other display devices, such as a solid-state electroluminescent display device. The outer surface of the viewing screen is the side of the screen facing the viewer.
Display tubes and devices must be suitable for use under high-intensity ambient-light conditions. Since in practice the brightness of a picture on a display tube is limited, the contrast must be as high as possible, so that even in the case of high-intensity ambient-light levels a clearly visible picture is presented to the viewer.
U.S. Pat. No. 3,504,212 describes a method of improving the contrast of a television picture tube in which a light-absorbing and light-scattering layer is provided between the phosphor layer and the inner surface of the faceplate. This layer serves to ensure that the ambient light which is incident via the faceplate is no longer reflected to the viewer. Before the light-absorbing layer is deposited the inner surface of the faceplate is frosted or roughened. This is achieved by immersion of the faceplate in an etching bath. This inevitably leads to the outer surface of the faceplate also being roughened. The profile on the outer surface of the faceplate as a result of etching has radii which cause a substantial scattering of light and an impermissible reduction in sharpness or definition and contrast of the picture to be reproduced on the faceplate. In order to preclude this, a further process step is applied in which both faceplate surfaces are heated and polished, so as to obtain a satin-type finish. Finally, the satin-finish outer surface of the faceplate is provided with an anti-reflective coating of magnesium fluoride to reduce residual reflections.
In mass-production processes etching is not a highly favoured process step because it requires special working conditions and because of the storage of the aggressive etching acids. Moreover, an additional process step is required after etching. Also, an additional coating must be applied, so that the method described in U.S. Pat. No. 3,504,212 is rather cumbersome.